Hi Refcyu,
We understand you'd like to view your mirrored content in landscape orientation. We're happy to help.
Based on the image you sent, it looks like you're holding your phone in the normal portrait orientation. To switch to landscape orientation, turn your phone so you're holding it that way.
Use AirPlay to stream video or mirror your device’s screen
Keep in mind not all applications will support a landscape orientation, when turning your phone. It will work for built in apps like Safari, Photos, Mail, etc. for example.
If the screen on your actual phone does not go in to landscape mode when rotating it, you'll want to make sure you have the orientation lock turned off.
Rotate the screen on your iPhone or iPod touch
We hope this helps. Take care!